Data storage system with complex memory and method of operating the same

ABSTRACT

A data storage system and a data storing method for the data storage system are provided. The data storage system includes a host unit, a storage unit, and a first input/output bus functioning as an interface between the host unit and the storage unit. The storage unit includes a non-volatile memory buffer unit and a flash memory unit. The non-volatile memory buffer unit includes a plurality of buffers arranged in parallel. The flash memory unit includes a plurality of data storage devices arranged in parallel to input and output data using a parallel method. In the method, a writing request is first classified into one of a plurality of grades according to a writing request frequency when there is a writing request and the writing requested data is stored in one of the non-volatile memory buffer unit and the flash memory unit according to the writing request frequency.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/485,346, filed Jul. 13, 2006 now U.S. Pat. No. 7,689,761 in the U.S.Patent and Trademark Office, the disclosure of which is incorporatedherein by reference. This application claims the priority benefit ofU.S. patent application Ser. No. 11/485,346, filed Jul. 13, 2006 in theU.S. Patent and Trademark Office. This application also claims thepriority benefit of Korean Patent Application No. 10-2005-0063301, filedon Jul. 13, 2005, and Korean Patent Application No. 10-2005-0076368,filed on Aug. 19, 2005 in the Korean Intellectual Property Office, thedisclosures of which are incorporated herein in their entirety byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a data storage system and method ofoperating the same, and more particularly, to a data storage system witha complex memory comprising a flash memory and a method of operating thedata storage system.

2. Description of the Related Art

Data storage systems using a flash memory device have been widely usedin an embedded system and a mobile system. The data storage system usinga flash memory device is one of electrically erasable programmableread-only memories (EEPROMs) on and from which the data can be written,read, and erased. While reading speed of the data storage system using aflash memory device is fast, writing and erasing speeds thereof areslow.

In the data storage system using a flash memory device, the datainput/output unit is a page of 2-Bytes or 4-Bytes for a NOR-type flashmemory device and 512-Bytes or 2-Kbytes for a NAND-type flash memorydevice. In addition, the erasing operation unit is a block of 128-Kbytesfor the NOR-type flash memory device and 16-Kbytes or 64-Kbytes for theNAND-type flash memory device.

That is, for the NOR-type flash memory device, it takes about 400 μs towrite 2-Bytes. For the NAND-type flash memory device, it takes about 220μs to write 512-Bytes and takes about 2 ms-2 s to erase 512-Bytes. Thatis, for the NAND-type memory device, the erasing operation is relativelyslow.

In contrast, for a random access memory (RAM) device, it takes severaltens of nano seconds to store a word. That is, the writing speed of datastorage systems using the flash memory device is far slower than that ofthe RAM device.

Accordingly, a flash memory device is generally used as a code memorydevice that does not frequently undergo writing operations or anauxiliary memory device of which writing performance is not important.In addition, a data storage system using a flash memory device cannotperform real-time data storing operation. Thus, there are limitations inusing the data storage systems using a flash memory device in a digitalproduct such as a digital camera or a digital camcorder.

Therefore, a technology for logically and effectively hiding the erasingoperation using a flash translation layer (FTL) employed using hardwareor software has been developed (refer to U.S. Pat. No. 6,311,290 to J.Kim, et al. and “A space-Efficient Flash Translation Layer for CompactFlash systems” IEEE Trans. Consumer Elec., Vol. 48, No. 2 pp, 366-375,2002).

However, even when the erasing operation is hidden using the FTL, thewriting speed of flash memory devices are still slow compared to RAMdevices. Thus, in the case of storing mass data, the writing speed of adata storage system using a flash memory is less than that of a RAMdevice. Due to this, a flash memory device is generally used as a codememory device that does not frequently undergo writing operations or asan auxiliary memory device of which writing performance is notimportant. Particularly, flash memory devices are very limited in beingused in a device such as the digital camera, camcorder and mobile phonethat process and store data in real-time.

An input/output buffer (or cash) is used to improve the input/outputperformance of the flash memory device. Korean Patent Application No.2003-32552 discloses a data storage system that uses an SRAM or DRAM asthe input/output buffer (or cash). US Patent Application No.2004/0193782 A1 discloses a data storage system that improves writingperformance by using a magnetic RAM as a writing buffer.

In the data storage system disclosed in Korean Patent Application No.2003-32552, input performance can be improved by maintaining data thatis frequently accessed in the cash, which has a fast input/output speed,using a temporal-spatial locality appearing in an input/output patternof the flash memory device. However, when the writing operation is doneonly in the cash, the data may be erased if power is turned off beforethe data is stored in the flash memory device. That is, the datamaintenance that is a basic condition of the data storage system cannotbe ensured. Therefore, although the reading performance is improved, theideal writing performance is not yet satisfied.

In the data storage system disclosed in US Patent Application No.2004/0193782 A1, when a writing operation larger than the capacity of aMRAM used in the system is continuously requested, an overflow isgenerated in the MRAM and thus the following writing request is directlydone in the flash memory device. This leads to a deterioration of thewriting performance.

SUMMARY OF THE INVENTION

The present invention provides a data storage system with a complexmemory that has an increased writing speed realized by managing onlydata which will be frequently updated in a writing buffer takingadvantage of spatial locality and can reduce the number of writing anderasing operations for the flash memory that is an advantage obtained byusing the buffer while reducing the size of the buffer.

The present invention also provides a method of operating the datastorage system.

According to an aspect of the present invention, there is provided adata storage system including: a host unit; a storage unit; and a firstinput/output bus functioning as an interface between the host unit andthe storage unit, wherein the storage unit includes a non-volatilememory buffer unit and a flash memory unit, the non-volatile memory unitincluding a plurality of buffers arranged in parallel, and the flashmemory unit including a plurality of data storage devices arranged inparallel to input and output data using a parallel method.

The number of buffers included in the non-volatile memory buffer unitmay be greater than the number of data storage devices included in theflash memory unit.

Each buffer may be a bank having a predetermined data storage capacity.

Each buffer may be a non-volatile memory chip.

Each data storage device may be a bank, and each bank may include aplurality of pages each having a predetermined data storage capacity.

Each data storage device may be a flash memory chip.

A data storage capacity of each buffer may be equal to, or N (N=1, 2, 3)times, that of each page.

A data storage capacity of each buffer may be equal to, or N (N=1, 2, 3)times, a block size that is a deleting unit of the flash memory.

According to another aspect of the present invention, there is provideda method of writing data in a data storage system including a host unit,a storage unit and a first input/output bus functioning as an interfacebetween the host unit and the storage unit, wherein the storage unitincludes a non-volatile memory buffer unit and a flash memory unit, thenon-volatile memory unit includes a plurality of buffers arranged inparallel and the flash memory unit including a plurality of data storagedevices arranged in parallel, the method including: writing data in afirst buffer of the plurality of buffers; writing data in a secondbuffer of the plurality of buffers; writing data in a third buffer ofthe plurality of the buffers; writing the data written in the firstbuffer in a first page of a first data storage device of the pluralityof data storage devices; and writing the data written in the secondbuffer in a first page of a second data storage device of the pluralityof the data storage devices, wherein the writing of the data in thesecond buffer and the writing of the data written in the first buffer inthe first page are simultaneously performed and the writing of the datain the third buffer and the writing of the data written in the secondbuffer in the first page are simultaneously performed.

The method may further comprise writing the data written in the thirdbuffer in a second page of the first data storage device of theplurality of data storage devices, wherein new data are written in thefirst buffer during the writing of the data written in the third bufferin a second page of the first data storage device of the plurality ofdata storage devices. The method may further comprise deleting the datawritten in the first buffer before the new data is written in the firstbuffer.

The number of buffers included in the non-volatile memory unit may begreater than the number of data storage devices included in the flashmemory unit. Each buffer may be a bank having a predetermined storagecapacity.

According to still another aspect of the present invention, there isprovided a method of writing data in a data storage system including ahost unit, a storage unit and a first input/output bus functioning as aninterface between the host unit and the storage unit, wherein thestorage unit includes a non-volatile memory buffer unit and a flashmemory unit, the non-volatile memory unit including a plurality ofbuffers arranged in parallel, and the flash memory unit including aplurality of data storage devices arranged in parallel, the methodincluding: writing data written in a first page of a first data storagedevice of the plurality of data storage devices in a first buffer of theplurality of buffers; writing data written in a first page of a seconddata storage device of the plurality of data storage devices in a secondbuffer of the plurality of buffers; writing data written in a secondpage of the first data storage device of the plurality of data storagedevices in a third buffer of the plurality of buffers; wherein thewriting of the data written in the first page of the second data storagedevice starts after the writing of the data written in the first page ofthe first data storage starts and ends after the writing of the datawritten in the first page of the first data storage device ends and thewriting of the data written in the second page of the first storagedevice starts after the writing of the data written in the first page ofthe first data storage device ends; and while the writing of the datawritten in the first page of the second data storage device in thesecond buffer is performed, the data written in the first buffer is readand while the writing of the data written in the second page of thefirst data storage device in the third buffer is performed, the datawritten in the second buffer is read.

The method may further include writing data written in a second page ofa second data storage device in the first buffer after the data writtenin the first buffer is read and before the writing of the data writtenin the second page of the first data storage device in the third bufferends.

According to still yet another aspect of the present invention, there isprovided a method of storing data in a data storage system having aflash memory unit and a writing buffer unit, the method including:classifying a writing request into one of a plurality of gradesaccording to a writing request frequency when a writing request is made;and storing the writing requested data in one of the memory unit and thewriting buffer unit according to the writing request frequency.

In the method, the storing of the writing requested data may comprisestoring a page having a relatively high writing request frequency in thewriting buffer unit, and, when the writing buffer unit is full, storingone of the pages stored in the writing buffer unit in the flash memoryunit.

When the writing request is classified into one of two grades, the datahaving a relatively high writing request frequency may be stored in thewriting buffer unit and the data having a relatively low writing requestfrequency are stored in the flash memory unit.

Alternatively, when the writing request is classified into one of twogrades, the storing of the writing requested data may include:temporarily storing data having a relatively high writing requestfrequency in a first writing buffer of the plurality of writing buffers;temporarily storing data having a relatively low writing requestfrequency in a second writing buffer of the plurality of writing buffersor the flash memory unit.

Alternatively, when the writing request is classified into one of threegrades, the storing of the writing requested data may include:temporarily storing data having a relatively high writing requestfrequency in a first writing buffer of the plurality of writing buffers;temporarily storing data having a middle writing request frequency in asecond writing buffer of the plurality of writing buffers; and storingdata having a relatively low writing request frequency in the flashmemory unit.

Alternatively, when the writing request is classified into one of threegrades, the storing of the writing requested data includes: temporarilystoring data having a relatively high writing request frequency in afirst writing buffer of the plurality of writing buffers; temporarilystoring data having a middle writing request frequency in a secondwriting buffer of the plurality of writing buffers; and temporarilystoring data having a relatively low writing request frequency in athird writing buffer of the plurality of writing buffers.

When there is a page for the writing request available in the writingbuffer unit, the page of the writing buffer unit may be updated to theavailable page in the writing buffer unit regardless of theclassification of the writing request.

The classifying of the writing request may include classifying thewriting request as a hot or cold pages using one or more LRU (LeastRecently Used) Q.

Alternatively, the classifying of the writing request may include addinga count value to the writing request and classifying the writing requestaccording to whether the count value is reduced to a predetermined valueafter a predetermined time has lapsed.

When the data having the relatively high writing request frequency is tobe written in the writing buffer unit and the writing buffer unit isfull, data stored in the writing buffer unit and having a relatively lowusing frequency may be stored in the flash memory, after which the datahaving the relatively high writing request frequency is stored in thewriting buffer unit.

After the data having the relatively high writing request frequency isstored in the writing buffer unit, metadata may be updated.

According to still yet another aspect of the present invention, there isprovided a data storage system including: a memory unit having a flashmemory unit and a writing buffer unit; and a memory control unitincluding a page classification unit for classifying a writing requestinto one of a plurality of grades according to a writing requestfrequency when a writing request is made, wherein the memory controlunit stores the writing requested data in one of the memory unit and thewriting buffer unit according to the writing request frequency.

When the writing request is classified into one of two grades, thememory control unit may store data having a relatively high writingrequest frequency in the writing buffer unit and store data having arelatively low writing request frequency in the flash memory unit.

The writing buffer unit may include a plurality of writing buffersarranged in parallel.

Alternatively, when the writing request is classified into one of twogrades, the memory control unit may temporarily store data having arelatively high writing request frequency in a first writing buffer ofthe plurality of writing buffers, temporarily store data having arelatively low writing request frequency in a second writing buffer ofthe plurality of writing buffers or in the flash memory unit.

Alternatively, when the writing request is classified into one of threegrades, the memory control unit may temporarily store data having arelatively high writing request frequency in a first writing buffer ofthe plurality of writing buffers, temporarily store data having a middlewriting request frequency in a second writing buffer of the plurality ofwriting buffers, and store data having a relatively low writing requestfrequency in the flash memory unit.

Alternatively, when the writing request is classified into one of threegrades, the memory control unit may temporarily store data having arelatively high writing request frequency in a first writing buffer ofthe plurality of writing buffers, temporarily store data having a middlewriting request frequency in a second writing buffer of the plurality ofwriting buffers, and temporarily store data having a relatively lowwriting request frequency in a third writing buffer of the plurality ofwriting buffers.

In the above-described systems and methods, the buffer may be one of abank and a non-volatile memory chip having a predetermined data storagecapacity. The first and second data storage devices may be first andsecond banks, respectively, and each of the first and second banks mayinclude a plurality of pages each having a predetermined data storagecapacity. The first and second data storage devices may be first andsecond flash memory chips, respectively.

The data storage capacity of the buffer may be equal to, or N (N=1, 2,3) times, that of the page. The data storage capacity of the buffer maybe equal to, or N (N=1, 2, 3) times, a block size that is a deletingunit of the flash memory.

The non-volatile memory unit and the flash memory unit may be formed ina single chip.

The storage unit may further include a flash translation layer and asecond input/output bus functioning as an interface between thenon-volatile memory buffer unit and the flash memory unit.

The storage unit may be provided in the form of a card that can bedetachably installed on the host unit.

The host unit may further include a first file system, a second filesystem, a flash translation layer interfacing with the first filesystem, and a device driver interfacing with the second file system.

The non-volatile memory chip may be selected from the group consistingof an MRAM, a PRAM, an FRAM, a non-volatile polymer memory chip, and acombination thereof.

The writing buffer unit may be a battery attached type non-volatilememory.

The writing buffer unit may be a volatile memory.

The flash memory unit may include a NAND-type Single Level Cell (SLC)flash memory and a NAND-type Multi Level Cell (MLC) flash memory.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of the present inventionwill become more apparent by describing in detail exemplary embodimentsthereof with reference to the attached drawings in which:

FIG. 1 is a block diagram of a data storage system having a complexmemory according to an embodiment of the present invention;

FIG. 2 is a block diagram of a storage unit of FIG. 1 according to anembodiment of the present invention;

FIG. 3 is a block diagram of a data storage system with a complex memoryaccording to another embodiment of the present invention;

FIG. 4 is a block diagram of a storage unit of FIG. 3 according to anembodiment of the present invention;

FIG. 5 is a block diagram illustrating a writing process in the datastorage system of FIG. 1 or 3 in reference to an applied clock cycleaccording to an embodiment of the present invention;

FIG. 6 is a block diagram illustrating a reading process in the datastorage system of FIG. 1 or 3 in reference to an applied clock cycleaccording to an embodiment of the present invention;

FIG. 7 is a block diagram illustrating a data storage system with acomplex memory according to another embodiment of the present invention;and

FIG. 8 is a flowchart illustrating a data storage method of the datastorage system of FIG. 7.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will now be described more fully with reference tothe accompanying drawings, in which exemplary embodiments of theinvention are shown.

FIG. 1 is a block diagram of a data storage system (hereinafter,referred as “first system”) having a complex memory according to anembodiment of the present invention.

Referring to FIG. 1, the first system includes a host unit 40, a storageunit 44 and a first I/O (input/output) bus 42. The first I/O bus 42functions as an interface between the host unit 40 and the storage unit44. The host unit 40 is a computer main body in which a file system 40 ais equipped. The storage unit 44 includes a flash translation layer 44a, a non-volatile buffer unit 44 b, a second I/O bus 44 c, and a flashmemory unit 44 d. The flash translation layer 44 a is provided in theform of hardware, which can identify data and a location where the datais recorded in the flash memory 44 d, perform the erasing operation, andidentify data and a location where the data is recorded in thenon-volatile memory buffer unit 44 b. Accordingly, the data recorded inthe flash memory buffer unit 44 b can be read using the flashtranslation layer 44 a. The second I/O bus 44 c functions as aninterface between the non-volatile memory buffer unit 44 b and the flashmemory unit 44 d. The storage unit 44 is portable and can be detachablyattached on the host unit 40. For example, the storage unit 44 may beany one of a compact flash card, a smart media card, a multimedia card,a secure digital card, a memory stick card, and a Universal Serial Bus(USB) storage device.

FIG. 2 is a block diagram of a storage unit of FIG. 1 according to anembodiment of the present invention. Referring to FIG. 2, thenon-volatile memory buffer unit 44 b includes first through nthnon-volatile memory buffers (hereinafter, referred as “buffers”) 44 b 1,44 b 2 . . . 44 b(n−1), 44 b(n). The first through nth buffers 44 b 1,44 b 2 . . . 44 b(n−1), and 44 b(n) are connected in parallel. The firstthrough nth buffers 44 b 1, 44 b 2 . . . 44 b(n−1), and 44 b(n) may bebanks or chips each having a predetermined capacity. The banks or chipsare MRAMs, FRAMs, PRAMs or non-volatile polymer memories. The flashmemory unit 44 d may include first through mth banks 44 d 1, 44 d 2 . .. 44 d(m−1), and 44 d(m). Each of the first through mth banks 44 d 1, 44d 2 . . . 44 d(m−1), and 44 b(m) includes a plurality of pages p1. Eachpage p1 has a predetermined capacity, which is preferably the same asthat of each of the first through nth buffers 44 b 1, 44 b 2 . . . 44b(n−1), 44 b(n) of the non-volatile memory buffer unit 44 b. That is, acapacity of one of the buffers forming the non-volatile memory bufferunit 44 b is identical to that of one of the pages p1 forming each ofthe first to mth banks 44 d 1, 44 d 2 . . . 44 d(m−1), and 44 d(m). Forexample, when the capacity of each buffer of the non-volatile memorybuffer unit 44 b is 512 bytes, the capacity of one page p1 which formseach of the first through mth banks 44 d 1, 44 d 2 . . . 44 d(m−1), and44 d(m) is also 512 bytes. Each of the first to mth banks 44 d 1, 44 d 2. . . 44 d(m−1), and 44 d(m) may be formed using individual flash memorychips. The flash memory unit 44 d may include a NAND-type Single LevelCell (SLC) flash memory and a NAND-type Multi Level Cell (MLC) flashmemory. At this point, each of the flash memories includes a pluralityof banks that are connected in parallel.

Although it is preferable that the capacity of each buffer of thenon-volatile memory buffer unit 44 b is same as that of each page p1 ofthe banks of the flash memory unit 44 d, the capacity of the buffer maybe n (where n=1, 2, 3) times larger or smaller than the capacity of thepage. Alternatively, the capacity of the buffer may be identical to or ntimes (n=1, 2, 3) the size of the block that is an erasing unit of theflash memory.

Meanwhile, in the nth buffer 44 b(n), the number “n” of the buffers (orbanks) contained in the non-volatile memory buffer unit 44 b may be 2p+1where the p represents a degree of the parallelism of the flash memorydevices of the flash memory unit 44 d. The degree of the parallelism isa natural number that is identical to or less than a value obtained bydividing the writing speed of a flash memory device by the writing speedof a non-volatile memory device. For example, when the first through nthbuffers 44 b 1, 44 b 2 . . . 44 b(n−1), 44 b(n) are FRAM devices and thefirst through mth banks 44 d 1, 44 d 2 . . . 44 d(m−1), and 44 d(m) ofthe flash memory unit 44 d are NAND-type flash memory devices, thewriting speed of the FRAM devices is 100 ns/word and the writing speedof the NAND-type flash memory devices is 500 ns/word. Therefore, thedegree “p” of the parallelism of the flash memory devices of the flashmemory unit 44 d in relation to the FRAM devices is 5. In this case, thenon-volatile memory buffer unit 44 b includes 11(2×5+1) buffers (orbanks) and the flash memory unit 44 d includes 5 logical I/O banks.

FIG. 3 is a block diagram of a data storage system (hereinafter,referred to as “second system”) having a parallel I/O complex memoryaccording to another embodiment of the present invention.

Referring to FIG. 3, the second system D includes a host unit 50, astorage unit 54 and an I/O bus 52. The host unit 50, I/O bus 52 andstorage unit 54 are equipped in a single main body. That is, while thestorage unit 44 of the first system is a portable device such as adiskette that can be detachably attached to the host unit 40, thestorage unit 54 and the host unit 50 of the second system may be chipsthat are mounted on a single board. The host unit 50 includes a filesystem 50 a and an additional file system 50 b. The host unit 50 furtherincludes a flash translation layer 50 c associated with the operation ofthe file system 50 a and a device driver 50 d associated with theoperation of the additional file system 50 b. The device driver 50 d isprovided in the form of software. In the second system D, the I/O bus 52functions as an interface between the host unit 50 and the storage unit54. The storage unit 54 includes a non-volatile memory unit 54 a and aflash memory unit 54 b. The flash memory unit 54 b may include aNAND-type SLC flash memory and a NAND-type MLC flash memory.

FIG. 4 is a diagram showing the storage unit 54 of the second system Dof FIG. 3 in more detail according to an embodiment of the presentinvention.

Referring to FIG. 4, the non-volatile memory buffer unit 54 a includesfirst through nth non-volatile memory buffers (hereinafter, referred as“buffers”) 54 a 1, 54 a 2 . . . 54 a(n−1), 54 a(n). The flash memoryunit 54 b may include first through mth banks 54 b 1, 54 b 2 . . . 54b(m−1), and 54 b(m). The buffers 54 a 1, 54 a 2 . . . 54 a(n−1), 54 a(n)are arranged in parallel and the banks 54 b 1, 54 b 2 . . . 54 b(m−1),and 54 b(m) are also arranged in parallel. The parallel arrangements ofthe buffers and banks means that their structures are parallel and theoperations thereof are performed in parallel. In this way, data can berecorded in the buffers and banks in parallel. The buffers 54 a 1, 54 a2 . . . 54 a(n−1), 54 a(n) may be identical to those of the firstsystem. Each of the banks 54 b 1, 54 b 2 . . . 54 b(m−1), and 54 b(m)includes a plurality of pages p2. Therefore, the banks 54 b 1, 54 b 2 .. . 54 b(m−1), and 54 b(m) may be identical to those of the firstsystem. The banks 54 b 1, 54 b 2 . . . 54 b(m−1), and 54 b(m) may beformed using a single chip. Alternatively, each of the banks 54 b 1, 54b 2 . . . 54 b(m−1), and 54 b(m) may be formed using individual flashmemory chips.

Meanwhile, each of the flash memory units 44 d and 54 d of the first andsecond systems includes a NAND-type flash memory or a NOR-type flashmemory. When the NOR-type flash memory is applied, data is input oroutput in units of bytes or words. In this case, it is difficult tomanage the data.

Therefore, in the present invention, even when the flash memory is theNOR-type, the pages of the banks of each of the flash memory units 44 dand 54 d may be logical pages conceptually identical to the case whenthe flash memory is the NAND-type. In addition, when the NOR-type flashmemory is applied to the flash memory units 44 d and 54 b, the size ofeach page of the banks may be identical to or different from that of thepage of the banks when the NAND-type flash memory is applied.

When the NOR-type flash memory is applied and the pages of the banks areidentical to that of the banks when the NAND-type flash memory isapplied, the operation of the flash memory units 44 d and 54 b isidentical to that when the NAND-type flash memory is applied, as will bedescribed later.

Writing processes of the first and second systems will now be describedwith reference to FIG. 5.

In FIG. 5, the arrow indicates a dataflow direction from thenon-volatile memory buffer unit to the flash memory unit. That is, thedata written in the non-volatile memory buffer unit is moved to theflash memory unit.

In FIGS. 5 and 6, it is assumed that one clock signal and two clocksignals are utilized for the reading and writing operations,respectively, of the non-volatile memory and two clocks and four clocksare utilized for the reading and writing operations, respectively, ofthe flash memory. The reading/writing speeds depend on the types of thenon-volatile memory and the flash memory. The relative ratio between thereading/writing performance of the non-volatile memory and thereading/writing performance of the flash memory becomes a design factorin determining the number of buffers of the non-volatile memory and thenumber of banks of the flash memory to be utilized.

In FIG. 5, an example is given for convenience showing that thenon-volatile memory buffer unit includes first to third buffers 70, 74and 76 that are arranged in parallel and the flash memory unit includesfirst and second banks 80 and 90. At this point, the first bank 80includes first through fifth pages 80 a, 80 b, 80 c, 80 d, and 80 e. Thesecond bank 90 includes first through fifth pages 90 a, 90 b, 90 c, 90d, and 90 e. The first through third buffers 70, 74 and 76 may be someof the buffers contained in the non-volatile memory buffer units 44 band 54 a as shown in FIGS. 2 and 4, respectively. The first and secondbanks 80 and 90 may be some of the banks contained in the flash memoryunits 44 d and 54 b as shown in FIGS. 2 and 4, respectively.

Referring to FIG. 5, data is recorded in the first buffer 70 of thenon-volatile buffer unit at a first clock pulse 1. The recording of thedata in the first buffer 70 is continued until a second clock pulse 2.After the recording of the data in the first buffer 70 is finished, thedata recorded in the first buffer 70 starts being recorded in the firstpage 80 a of the first bank 80 at a third clock pulse 3. The process forrecording the data recorded in the first buffer 70 in the first page 80a of the first bank 80 is continued until a sixth clock pulse. The timespent in recording the data recorded in the first buffer 70 in the firstpage 80 a of the first bank 80 is two times that spent in recording thedata in one of the buffers of the non-volatile memory buffer unit.Therefore, while the data recorded in the first buffer 70 is recorded inthe first page 80 a of the first bank 80, further data cannot berecorded in the first buffer 70. Therefore, while the data recorded inthe first buffer 70 is recorded in the first page 80 a of the first bank80, further data input to the non-volatile memory buffer unit isconsecutively recorded in the second and third buffers 74 and 76. Thepoint of time when data is recorded in the second and third buffers 74and 76 depends on whether the data input to the non-volatile buffer unitis continuous or discontinuous. For example, if there is no data inputto the non-volatile memory buffer unit until the data recorded in thefirst buffer 70 moves completely to the first page 80 a of the firstbank 80, the following data input to the non-volatile memory buffer unitcan be recorded again in the first buffer 70. On the other hand, if datais continuously input, further input data is recorded in the secondbuffer 74 after initial data is first input to the first buffer 70. Thatis, the recording of data to the second buffer 74 starts at the thirdclock pulse 3 and ends at a fourth clock pulse 4. The recording of thedata recorded in the first buffer 70 to the first page 80 a of the firstbank 80 is continued at the fourth clock pulse 4 where the recording ofthe data to the second buffer 74 is completed. After recording of thedata to the second buffer 74 is completed, the data recorded in thesecond buffer 74 starts being recorded in the first page 90 a of thesecond bank 90. A process for recording the data recorded in the secondbuffer 74 in the first page 90 a of the second bank 90 starts at thefifth clock pulse 5 and continues until an eighth clock pulse 8. Aprocess for recording the data recorded in the first buffer 70 in thefirst page 80 a of the first bank 80 starts at the third clock pulse 3and continues until the sixth clock pulse 6. Therefore, during the fifthand sixth clock pulses 5 and 6, recording of the data to the first andsecond buffers 70 and 74 cannot be done. Recording of the data to thesecond buffer 74 cannot be done until the eighth clock pulse 8.Therefore, after the data is recorded in the second buffer 74, datainput to the non-volatile memory buffer is recorded in the third buffer76 at the fifth clock pulse 5. Recording of the data in the third buffer76 continues until the sixth clock pulse 6. At the sixth clock pulse 6,the recording of the data to the third buffer 76 is completed and thedata recorded in the first buffer 70 is recorded in the first page 80 aof the first bank 80. However, the data recorded to the second buffer 74is continuously recorded in the first page 90 a of the second bank 90.As described above, at the sixth clock pulse 6 the recording of data tothe third buffer 76 is completed and the first buffer 70 of thenon-volatile memory buffer unit is empty. Therefore, further data beinginput to the non-volatile memory buffer unit at the seventh clock pulse7 can be recorded to the first buffer 70.

The reading operation of the first or second system will now bedescribed.

The reading operation of the first or second system is similar to aconventional data cashing method. That is, an address region that isfrequently used is maintained in a part of the non-volatile RAM usingthe hardware cash or the LRU (Least Recently Used) software. When thereis a reading request for the data remained in the non-volatile RAM, thedata is quickly read from the non-volatile RAM without using the flashmemory and transmitted. According to a feature of the present invention,a prefetching method is additionally utilized for the reading operation.In this case, the stored data is quickly read from the selected bank ofthe flash memory using the parallel structure and the prefetching of thedata is done from the selected buffer of the non-volatile memory bufferunit.

In the above-described reading method, as in the writing operation,using the degree of the parallelism corresponding to the relative ratioof the reading performance between of the non-volatile RAM and the flashmemory utilized, data that is expected to be read is first loaded on theselected buffer of the non-volatile memory buffer unit and data that isalready read is pushed out.

When the estimation of data that is expected to be read is accurate, thereading speed may be identical to a case where data is read from thenon-volatile RAM even when a buffer having a relatively small capacityis used.

FIG. 6 shows the reading process of the first or second system inreference to an applied clock cycle. In FIG. 6, the arrow indicates adataflow direction.

Referring to FIG. 6, data recorded in the first page 80 a of the firstbank 80 of the flash memory unit is read and recorded in the firstbuffer 70 of the non-volatile memory buffer unit (hereinafter, “firstrecording”). At this point, the bank 80 may be one of the bankscontained in the SLC flash memory of the flash memory unit or one of thebanks contained in the MLC flash memory of the flash memory unit. Sincereading speed of the flash memory unit is faster than writing speed ofthe flash memory unit, the first recording starts at the first clockpulse 1 and continues until the second clock pulse 2. While the firstrecording is being performed, the data recorded in the first page 90 aof the second bank 90 are read and recorded in the second buffer 74 ofthe second non-volatile memory buffer unit (hereinafter, “secondrecording”). The second recording starts one clock pulse after the firstrecording starts. That is, the second recording starts at the secondclock pulse 2 and continues until the third clock pulse 3. While thesecond recording is being performed, data recorded in the second page 80b of the first bank 80 is read and recorded in the third buffer 76(hereinafter, “third recording”). The third recording starts at thethird clock pulse 3 and continues until the fourth clock pulse 4.

The recording of data in the first and third buffers 70, 74 and 76 ofthe non-volatile buffer unit from the flash memory unit through thefirst through third recording processes can be successively read. Thatis, the data recorded in the first buffer 70 is read right after thedata recorded in the first page 80 a of the first bank 80 is written inthe first buffer 70. That is, the data transferred from the first page80 a of the first bank 80 to the first buffer 70 during the firstrecording is read until the third clock pulse 3. When the third clockpulse 3 appears, the second recording where the data recorded in thefirst page 90 a of the second bank 90 is recorded in the second buffer74 is completed. Therefore, right after the data recorded in the firstbuffer 70 is read until the third clock pulse 3, the data recorded inthe second buffer 74 can be read at the fourth clock pulse 4. Likewise,the data recorded in the second buffer 74 is read at the fourth clockpulse 4, the data recorded in the third buffer 76 can be read at thefifth clock pulse 5.

FIG. 7 shows a block diagram of a data storage system (hereinafter,“third system”) with a complex memory according to another embodiment ofthe present invention.

Referring to FIG. 7, the third system includes a data storage unit 100and a host 110. The data storage unit 100 includes an I/O interface unit120, a flash translation layer 130, and a memory unit M1. The memoryunit M1 includes a writing buffer unit 140 and a flash memory 150. Thedata storage unit 100 is portable and can be detachably attached on thehost 110. For example, the data storage unit 100 may be any one of acompact flash card, a smart media card, a multimedia card, a securedigital card, a memory stick card, and a USB storage device. The host110 controls the flash memory unit 150 through a memory control unit 131of the flash translation layer 130. The host 110 transfers a logicaladdress designating a location in which data of the flash memory will berecorded to the memory flash translation layer 130. The host 100 may bea personal computer, a digital camera, or other portable electronicdevices such as an MP3 player.

The I/O interface unit 120 functions as an interface between the host110 and the data storage unit 100 for data exchange between eachfunctional part of the data storage unit 100 and the host 110. The I/Ointerface unit 120 transfers the logical address designating a readtarget page to the flash translation layer 130 by decoding a flashmemory reading order transferred from the host 110. In addition, the I/Ointerface unit 120 transfers read target data, which is transferred fromthe flash memory unit 150 by the memory control unit 131, to the host110 through an external bus. The I/O interface unit 120 transfers thelogical address designating a write target page to the flash translationlayer 130 by decoding a flash memory writing order transferred from thehost 110.

The flash translation layer 130 functions to allow the flash memory unit150 to be used as a block device such as a hard disk. The flashtranslation layer 130 functions to translate the logical address intothe physical address using mapping information when there is awriting/reading operation request for at the logical address whilemaintaining the mapping information between the logical address on theimaginary block device and the physical address on the flash memory.Particularly, when there is a writing operation request for translatingdata of a logical address, the flash translation layer 130 functions toelectrically delete and rerecord the physical address corresponding tothe logical address or to remap the logical address into anotherphysical address. The flash translation layer 130 may include a memorycontrol unit 131 and a RAM 132. The memory control unit 131 may includea page classification unit 133 for classifying a writing request intoone of a plurality of grades. The memory control unit 131 stores alogical address for the writing request received through the I/Ointerface unit 110. When the writing request is received through the I/Ointerface unit 110, the page classification unit 133 classifies thewriting request into one of the plurality of grades according to awriting request frequency. The page classification unit 133 may classifythe writing request into two or three grades, as will be described inmore detail.

The memory control unit 131 stores data requested to be written in thewriting buffer unit 140 or the flash memory unit 150 according to agrades classified by the page classification unit 133. If the writingrequest can be classified into one of two grades, the memory controlunit 131 temporarily stores the data having a grade of a high writingrequest frequency in the writing buffer unit 140 and stores the datahaving a grade of a low writing request frequency in the flash memory150.

The data received through the I/O interface unit 120 is temporarilystored in the writing buffer unit 140 in a unit of a page by the controlof the memory control unit 131. The writing buffer unit 140 may be anon-volatile memory selected from the group consisting of an MRAM, anFRAM, a PRAM, a polymer memory, and a combination thereof. In addition,the writing buffer unit 140 may be a battery attached type non-volatilememory. Alternatively, the writing buffer unit 140 may be a volatilememory. The writing buffer unit 140 may be formed of a plurality ofbanks or chips that are arranged in parallel.

The flash memory unit 150 is divided into a plurality of physical blockseach having a predetermined size. The physical blocks have their ownphysical block number (0, 1, 2, . . . n−1). Therefore, they can beindividually designated and used as a deleting unit. In addition, eachof the physical blocks is divided into a plurality of physical pageseach having a predetermined size. Each of the physical pages can beaccessed as an offset of a corresponding physical block and used as areading/writing unit. The physical block and the physical page may beset to have various sizes according to the size of the NAND-type flashmemory. For example, when an overall capacity, block size and page sizeof the flash memory unit 150 are 128 Mbytes, 16 Kbytes, and 512 Bytes,respectively, each physical block of the flash memory comprises 32physical pages. The flash memory unit 150 may include a NAND-type SLCflash memory 150 a and a NAND-type MLC flash memory 150 b. At thispoint, each of the flash memories 150 a and 150 b has a plurality ofphysical blocks.

A process for storing data in the third system will now be described.

The present invention is based on a fact that the number of writingoperations and the buffer size can be reduced by using a temporallocality appearing in the writing operation.

There is a temporal-spatial locality in a memory accessing pattern of acomputer system. Likewise, there is also the temporal-spatial localityin the I/O pattern of the storage system. At this point, the temporallocality means that data that has been more recently accessed is morelikely to be accessed again. The spatial locality means that a block,file, or directory that is adjacent to a block, file or directory thathas been recently accessed is more likely to be accessed in the future.

That is, similar to a theory of 20:80, a part of the address space isfrequently requested for the writing operation and the rest of theaddress space is rarely requested for the writing operation once thewriting request is done. The part of the address space that isfrequently requested for the writing operation is called a hot page andthe rest of the address space that is rarely requested for the writingoperation are each called a cold page. The classification of the addressspaces as hot or cold pages and the storing of the classified data willnow be described in more detail with reference to FIG. 8.

FIG. 8 is a flowchart illustrating a data storage method of the datastorage system of FIG. 7.

Referring to FIGS. 7 and 8, the memory control unit 131 receives a datawriting request from the host 110 through the I/O interface unit 120(Step 200). At this point, data is transferred in a unit of a page andthe memory control unit 131 receives a logical address designating awrite target page. The memory control unit 131 stores the logicaladdress in the RAM 132. The memory control unit 131 determines if thereis a write target page having an address identical to that of the writetarget page for the writing request in the writing buffer unit 140 ofthe memory unit M1 (Step 210). When it is determined there is a writetarget page having an address identical to that of the write target pagefor the writing request in the writing buffer unit 140 of the memoryunit M1, the memory control unit 131 updates the address of the writingbuffer unit 140 regardless of the page classification (Step 220).

When it is determined there is no write target page having an addressidentical to that of the write target page for the writing request inthe writing buffer unit 140 of the memory unit M1, the pagespecification unit 133 classifies the writing request as a hot or coldpage according to the frequency of the writing request (Step 230). Thatis, the page classification unit 133 estimates the probability of anamendment using the temporal locality appearing in the writing operationand classifies the writing request as a hot or cold page with referenceto the estimation. The page classification is done according to thefollowing method.

First, the writing request may be classified by a method using the LRUsoftware. This method is based on the temporal locality that a page thatis not accessed for a long time is less likely to be accessed in thefuture. That is, one LRU Queue (hereinafter “LRU Q”) is managed and itis determined if an address region corresponding to a new writingrequest is registered in the LRU Q when the new writing request isinput. When it is determined that an address region corresponding to anew writing request is registered in the LRU Q, it is regarded thatthere was a writing request for the address region during the formerwriting operation. Therefore, the writing request is classified as a hotpage. When it is determined that the address region corresponding to anew writing request is not registered in the LRU Q, the writing requestis classified as a cold page. The classified writing request is added tothe LRU Q.

Second, the writing request may be classified by a method adding a countvalue to the address receiving the writing request. At a time when thewriting request is generated, a specific count value is set and thespecific count value gradually reduces as time elapses. When apredetermined time (e.g., 10 minutes) has lapsed, the count value is setas “0”. At this point, the writing request generated in the region wherethe count value is not “0” is classified as a hot page and the writingrequest generated in the region where the count value is “0” isclassified as a cold page.

The writing request can be classified by the locality extracted by avariety of other methods such as a method using a hardware cashstructure.

Although the writing request is classified into two grades such as hotand cold pages according to the frequency in the foregoing description,the present invention is not limited to this case. That is, the writingrequest may be classified into three grades such as high, middle and lowgrades according to the frequency.

Referring again to FIG. 8, it is determined if the writing request isclassified as a hot page (Step 240). When it is determined that thewriting request is classified as a cold page, the memory control unit131 directly stores the writing requested data classified as a cold pagein the flash memory unit 150 without temporarily storing the data in thewriting buffer unit 140 (Step 250). At this point, the writing requesteddata may be stored in the MLC flash memory 150 b of the flash memoryunit 150. Alternatively, the writing requested data may be stored in theflash memory unit 150 via the writing buffer unit 140 when the writingbuffer unit 140 is used to improve the performance. As a result, thenumber of deleting operations performed on the flash memory unit 150 canbe reduced.

In Step 240, when it is determined that the writing request isclassified as a hot page, the memory control unit 131 determines if thewriting buffer unit 140 is full (Step 260). When it is determined thatthe writing buffer unit 140 is full, one of the pages of the writingbuffer unit 140, which has the lowest locality (i.e., the lowest usingfrequency), is stored in the flash memory unit 150 (Step 270). At thispoint, the page having the lowest locality may be stored in the SLCflash memory 150 a of the flash memory unit 150.

That is, one of the pages stored the writing buffer unit 140, which wasaccessed the longest time ago, is selected and the selected page isstored in the SLC flash memory 150 a of the flash memory unit 150. Then,the writing request is stored in a range, in which the accessed page wasstored, of the writing buffer unit 140.

Then, when the writing buffer unit 140 is not full or an additionalspace is generated in the writing buffer unit 140 in Step 270, thewriting requested data are stored in the writing buffer unit 140 (Step280). For example, when it is assumed that the writing request isgenerated in order of pages A1, B, A2, C, D, E, F, G, H, and A3 for theaddress spaces A through Z and the address space A is classified as thehot page and the pages B through H are classified as the cold pages, thewriting request for the pages A1, A2 and A3 can be processed in thewriting buffer even if the writing buffer has a size that can store onlyone flash page. Therefore, an advantage that only the page A3 is storedin the flash memory unit can be actually realized.

Alternatively, when the writing buffer unit 140 includes a plurality ofwriting buffers, the data classified as a hot page can be stored in afirst writing buffer and the data classified as a cold page can bestored in a second writing buffer. Then, the data classified as the coldpage and stored in the second writing buffer can be stored in the SLCflash memory 150 a of the flash memory unit 150.

Alternatively, when the page classification unit 133 classifies thewriting requested data into three grades, the memory control unit 131stores the data having high, middle and low using frequencies in firstand second writing buffers and the MLC flash memory 150 b of the flashmemory unit 150, respectively.

Alternatively, the memory control unit 131 temporarily stores the datahaving high, middle and low using frequencies in first, second and thirdwriting buffers, respectively. Then, the data stored in the thirdwriting buffer moves to and is stored in one of the SLC or MLC flashmemories 150 a and 150 b of the flash memory unit 150.

As described above, after the writing requested data is stored in thewriting buffer unit 140 and the metadata is updated (Step 290).

In the data storage system according to the present invention, the datais written or read using a parallel input/output method by arranging anappropriate number of non-volatile memory buffers and flash memory banksin parallel considering a relative operation speed between anon-volatile memory device and a flash memory device. Therefore, a largecapacity of data can be continuously processed, thereby preventingoverflow and increasing writing/reading speed.

In addition, data deletion caused by unexpected accidents such as powerbeing turned off when some data has not been stored in the flash memorydevice can be prevented, thereby stably maintaining the data.

Furthermore, the non-volatile memory buffer unit can be used as the cashfor the reading operation and thus the XIP (eXecuition In Place) supportcan be added to the NAND-type flash memory by supporting the datainput/output in a unit of a byte.

In addition, the data classified as a hot page (having a high writingrequest frequency) is recorded in the writing buffer (e.g., anon-volatile RAM) and the data classified as a cold page (having a lowwriting request frequency) is recorded in the flash memory unit.Therefore, writing speed increases. Furthermore, since the writingbuffer is formed of a non-volatile RAM such as an MRAM, a PRAM, an FRAMand the like, data can be permanently stored and be read and writehaving a capacity (e.g., 8-byte) less than the page unit of the flashmemory. In addition, since the flash memory includes the SLC and MLCflash memories, memory efficiency and function can be enhanced. Sinceonly data classified as a cold page is recorded in the flash memoryunit, the number of writing operations for the flash memory can bereduced and thus the number of deleting operations can also be reduced.The reduction in the number of the deleting operations increases theservice life of the flash memories.

While the present invention has been particularly shown and describedwith reference to exemplary embodiments thereof, it will be understoodby those of ordinary skill in the art that various changes in form anddetails may be made therein without departing from the spirit and scopeof the present invention as defined by the following claims.

For example, additional components may be further added between the hostunit and the storage unit. In addition, other types of the non-volatilememory chips can be used to form the writing buffer unit. The writingbuffer units may include different types of non-volatile memory devices.In addition, the flash memory unit may include only one of the SLC andMLC flash memories.

What is claimed is:
 1. A method of storing data in a data storage systemhaving a flash memory unit and a writing buffer unit, the methodcomprising: classifying a writing request into one of a plurality ofgrades according to a writing request frequency at a time a writingrequest is made; and storing the classified writing requested data inone of the flash memory unit and the writing buffer unit according tothe writing request frequency, wherein the writing buffer unit includinga plurality of buffers arranged in parallel and the flash memory unitincluding a plurality of data storage devices, wherein a number ofbuffers included in the writing buffer unit is greater than a number ofdata storage devices included in the flash memory unit.
 2. The method ofclaim 1 wherein, when the writing request is classified into one of twogrades, the data having a relatively high writing request frequency isstored in the writing buffer unit and the data having a relatively lowwriting request frequency is stored in the flash memory unit.
 3. Themethod of claim 2, wherein the flash memory unit is one of a NAND-typeSLC flash memory and a NAND-type MLC flash memory.
 4. The method ofclaim 2, wherein, when the data having the relatively high writingrequest frequency is to be written in the writing buffer unit and thewriting buffer unit is full, data stored in the writing buffer unit andhaving a relatively low using frequency is stored in the flash memory,after which the data having the relatively high writing requestfrequency is stored in the writing buffer unit.
 5. The method of claim4, wherein, after the data having the relatively high writing requestfrequency is stored in the writing buffer unit, metadata is updated. 6.The method of claim 2, wherein, after the data having the relativelyhigh writing request frequency is stored in the writing buffer unit,metadata is updated.
 7. The method of claim 1, wherein, when the writingrequest is classified into one of two grades, the storing of the writingrequested data comprises: temporarily storing data having a relativelyhigh writing request frequency in a first writing buffer of theplurality of writing buffers; and temporarily storing data having arelatively low writing request frequency in a second writing buffer ofthe plurality of writing buffers or in the flash memory unit.
 8. Themethod of claim 7, wherein the flash memory unit is one of a NAND-typeSLC flash memory and a NAND-type MLC flash memory.
 9. The method ofclaim 1, wherein, when the writing request is classified into one ofthree grades, the storing of the writing requested data comprises:temporarily storing data having a relatively high writing requestfrequency in a first writing buffer of the plurality of writing buffers;temporarily storing data having a middle writing request frequency in asecond writing buffer of the plurality of writing buffers; and storingdata having a relatively low writing request frequency in the flashmemory unit.
 10. The method of claim 9, wherein the flash memory unit isone of a NAND-type SLC flash memory and a NAND-type MLC flash memory.11. The method of claim 1, wherein, when the writing request isclassified into one of three grades, the storing of the writingrequested data comprises: temporarily storing data having a relativelyhigh writing request frequency in a first writing buffer of theplurality of writing buffers; temporarily storing data having a middlewriting request frequency in a second writing buffer of the plurality ofwriting buffers; and temporarily storing data having a relatively lowwriting request frequency in a third writing buffer of the plurality ofwriting buffers.
 12. The method of claim 11, wherein the flash memoryunit is one of a NAND-type SLC flash memory and a NAND-type MLC flashmemory.
 13. The method of claim 1, wherein, when there is a page for thewriting request available in the writing buffer unit, the page of thewriting buffer unit is updated to the available page in the writingbuffer unit regardless of the classification of the writing request. 14.The method of claim 1, wherein the classifying of the writing requestcomprises classifying the writing request as a hot or cold page usingone or more LRU (Least Recently Used) queue.
 15. The method of claim 1,wherein the classifying of the writing request comprises adding a countvalue to the writing request and classifying the writing requestaccording to whether the count value is reduced to a predetermined valueafter a predetermined time has lapsed.
 16. The method of claim 1,wherein the storing of the writing requested data comprises storing apage having a relatively high writing request frequency in the writingbuffer unit, and, when the writing buffer unit is full, storing one ofthe pages stored in the writing buffer unit in the flash memory unit.17. The method of claim 16, wherein, after the data having therelatively high writing request frequency is stored in the writingbuffer unit, metadata is updated.
 18. The method of claim 1, wherein thewriting buffer is one selected from the group consisting of an MRAM, aPRAM, an FRAM, and a non-volatile polymer memory chip, or a combinationthereof.
 19. The method of claim 1, wherein the writing buffer unit is abattery attached type non-volatile memory.
 20. The method of claim 1,wherein the flash memory unit includes a NAND-type SLC flash memory anda NAND-type MLC flash memory.
 21. A data storage system comprising: amemory unit having a flash memory unit and a writing buffer unit; and amemory control unit including a page classification unit for classifyinga writing request into one of a plurality of grades according to awriting request frequency at a time a writing request is made, whereinthe memory control unit stores the classified writing requested data inone of the flash memory unit and the writing buffer unit according tothe writing request frequency, wherein the writing buffer unit includinga plurality of buffers arranged in parallel and the flash memory unitincluding a plurality of data storage devices, wherein a number ofbuffers included in the writing buffer unit is greater than a number ofdata storage devices included in the flash memory unit.
 22. The datastorage system of claim 21, wherein, when the writing request isclassified into one of two grades, the memory control unit stores datahaving a relatively high writing request frequency in the writing bufferunit and stores data having a relatively low writing request frequencyin the flash memory unit.
 23. The data storage unit of claim 22, whereinthe flash memory unit is one of a SLC flash memory and a MLC flashmemory.
 24. The data storage system of claim 21, wherein, when thewriting request is classified into one of two grades, the memory controlunit temporarily stores data having a relatively high writing requestfrequency in a first writing buffer of the plurality of writing buffers,and temporarily stores data having a relatively low writing requestfrequency in a second writing buffer of the plurality of writing buffersor in the flash memory unit.
 25. The data storage unit of claim 24,wherein the flash memory unit is one of a SLC flash memory and a MLCflash memory.
 26. The data storage system of claim 21, wherein, when thewriting request is classified into one of three grades, the memorycontrol unit temporarily stores data having a relatively high writingrequest frequency in a first writing buffer of the plurality of writingbuffers, temporarily stores data having a middle writing requestfrequency in a second writing buffer of the plurality of writingbuffers, and stores data having a relatively low writing requestfrequency in the flash memory unit.
 27. The data storage system of claim21, wherein, when the writing request is classified into one of threegrades, the memory control unit temporarily stores data having arelatively high writing request frequency in a first writing buffer ofthe plurality of writing buffers, temporarily stores data having amiddle writing request frequency in a second writing buffer of theplurality of writing buffers, and temporarily stores data having arelatively low writing request frequency in a third writing buffer ofthe plurality of writing buffers.
 28. The data storage unit of claim 27,wherein the flash memory unit is one of a SLC flash memory and a MLCflash memory.
 29. The data storage unit of claim 21, wherein the writingbuffer unit is one selected from the group consisting of an MRAM, aPRAM, an FRAM, and a non-volatile polymer memory chip, or a combinationthereof.
 30. The data storage unit of claim 21, wherein the writingbuffer unit is a battery attached type non-volatile memory.
 31. The datastorage unit of claim 21, wherein the writing buffer unit is a volatilememory.
 32. The data storage unit of claim 21, wherein the flash memoryunit is one of a SLC flash memory and a MLC flash memory.